1. Technical Field
The technology described herein is generally related to the field of electrical circuits and power supply systems and, more particularly, to a system in which multiple power supplies, having switching modes of operation, are connected to a given load.
2. Description of Related Art
For many applications, it may be preferred that a load be powered from multiple power supplies working in concert rather than from one, more powerful, supply. This technique is generally referred to in the art as “load sharing.” High output power supplies are generally inefficient and slow. Smaller, common-use-sized, power supplies generally are more easily obtained, repaired and cost efficient.
In a load-shared, switching mode system, the multiple power supplies connected to the load are equipped to communicate with one another as to how much load demand each is supplying, generally to the purpose of sourcing approximately equal shares of the load current. There are several scenarios where load sharing is preferable.
In high-reliability or high-up-time load systems, it may be desired that redundant power supplies be available for increasing mean-time-between-failure (MTBF). Power may be supplied by multiple supplies, each running at a fraction of capacity, which allows the load to continue to operate even when one supply fails.
In low-noise systems, it may be desirable to run switching power supplies with their respective clocking out-of-phase to produce a composite power line to the load with reduced ripple current and noise.
A common approach to load-sharing systems uses switching power supplies as the basic building blocks because each has a generally high efficiency, small size per watt of output, and low weight. FIG. 1 (Prior Art) shows a typical shared “Load” system. The output current “ILD2” of the Power Supply 2 “Slave” tracks the output current “ILD2” of the Power Supply 1 “Master.”
It is known that as well as being a power source, a commercial switching power supply can also become a current sink. When switching power supplies and controls are used in a commonly known master-slave configuration as shown in FIG. 1 (Prior Art), there is a need to prevent a condition in which any one of the power supplies sinks current from another. This phenomenon is referred to as “current recirculation,” where electrical current is drawn from a first unregulated source by one supply system and absorbed and returned to a second unregulated source supply system. Typical problems resultant from this phenomenon are illustrated by FIGS. 2 and 3. FIG. 2 (Prior Art Problem) illustrates increased power dissipation due to recirculation of current in a two unregulated power sources system. FIG. 3 (Prior Art Problem) illustrates a recirculation condition where current is returned to a source and the source is thereby lifted in voltage which could cause an over-voltage fault condition to be triggered. In other words, if the power supplies are connected in the common current-tracking master-slave configuration where the slave tries to match its output current to that of the master, then at low load currents, voltage offset in the tracking circuitry of the slave may in fact command the slave to run a negative current, that is, the slave may sink current. It could also be that the master sinks current from the slave.
One technique for attempting to prevent current recirculation is to use power supplies that cannot sink current. An example of this approach would be a load-sharing design using linear power supplies with emitter follower outputs. This approach has a drawback in that such power supplies are increasingly inefficient the greater the unregulated voltage is relative to the load voltage. Another technique is to build load-sharing power supply systems which use switching supplies that only operate in a discontinuous mode—generally defined as not allowing inductor current to flow when the direction of current flow would be from a load back into a power supply. This approach is not power-efficient and has a poorer transient response than non-discontinuous mode supplies. Still another approach tries to minimize current recirculation by putting limits on the offset voltage in the slave amplifier.
There is a need for better load-shared, switching mode, power supply systems.